<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>


        /* 特殊：父元素不设置height，它会获取子元素的高度  */


    .parent{
        width: 200px;
        background: red;


        
    }

    .child{
        width: 50px;
        height: 200px;
        background-color: black;

    }
    </style>
</head>
<body>

    <div class="parent">
<div class="child">


</div>
    </div>
    
</body>
</html>